Accurate measurement of pH is important in diverse fields such as process control, reaction kinetics, environmental and biomedical research and oilfield applications. Many chemical processes require pH monitoring and control at extreme conditions of temperature, pressure, and salinity. However, as will be described below, standard potentiometric techniques provide accurate measurements at moderate temperatures, pressures, and salinities. Measurements of pH of standard buffers at high-temperature and high-pressure using hydrogen and/or glass electrodes have been reported by LePeintre, Bull. Soc. Franc. Electr. 1960, 8, 584, and Kryukov, et al. as cited in pH Measurement: Fundamentals, Methods, Applications, Instrumentation VCH Publishers, 1991; however, liquid junction instability results in uncertainties in the measurement. Furthermore, pressure balancing needs and liquid junctions make it practically inconvenient to use hydrogen and/or glass electrodes for routine measurements in high-pressure, high-temperature systems. Boreng, et al. in SPE European Formation Damage Conference, May 13-14, 2003, The Hague, The NetherlandsSPE 82199 describe a solid-state electrode for high-temperature and high-pressure pH measurement. While this proposed method eliminates the liquid junction uncertainty, the pH is measured relative to sodium activity that must be independently determined to determine the absolute pH.
Spectroscopic measurement of pH with very high accuracy using pH-sensitive dyes has been a well-established laboratory technique at ambient conditions since the early 1900's (Bates, Determination of pH: Theory and Practice, Chapter 6, John Wiley, 1964). More recently, this technique has been shown to improve precision for seawater and freshwater pH measurements over a range of ionic strengths where potentiometric techniques can prove to be problematic (Yao, et al., Environ. Sci. Technol., 2001, 35, 1197-1201; Martz, et al. Anal. Chem., 2003, 75, 1844-1850). These references cite the advantages of the spectroscopic technique with respect to low drift, reproducibility, and rapidness of the measurement as compared to the standard glass electrodes. Furthermore, because pH measurement depends only on the molecular properties of the indicator dyes, once the dye equilibrium dissociation constants have been characterized, the need for calibration prior to every measurement is eliminated. The methods described above allow implementation of the spectroscopic technique at close to ambient conditions and narrow ionic strength intervals corresponding to either seawater or fresh water conditions, however, the methods do not allow for implementation of the spectroscopic technique at high-temperatures and/or high-pressures.
Because of the lack of robust high-temperature high-pressure pH measurement techniques, currently high-temperature and high-pressure aqueous system equilibrium and the role of pH is characterized using chemical modeling of complex chemical equilibria to calculate the pH. In oilfields, it is important to know the pH of formation fluid to predict corrosion rates, scale formation, water compatibility, etc. Current practice involves collecting fluid samples in single-phase bottles, bringing them to surface and flashing them. The pH at high-temperature high-pressure downhole conditions is obtained by simulations that use ambient flashed gas and water phase analysis as inputs to chemical equilibrium models. This introduces errors in pH measurement due to sample handling, precipitation of ionic solids from flashed water samples and modeling uncertainties of complex ionic equilibria.
Spectroscopic measurement of pH relies on pH-sensitive dyes that can exist in an acid or base form. The optical absorbance spectra of pH-sensitive dyes change as they convert from their acid (A) to base form (B):AB+H+  Eq. 1The fraction of the dye present in the acid and base forms depends on the pH of the solution. The pH is calculated using the following equation:
                              pH          =                                    pK              a                        +                          log              ⁢                                                          ⁢                                                γ                  B                                                  γ                  A                                                      +                          log              ⁢                                                          ⁢                                                [                  B                  ]                                                  [                  A                  ]                                                                    ,                            Eq        .                                  ⁢        2            where pKa is −log10Ka;
Ka is the thermodynamic equilibrium constant for the dye and is a function of temperature and pressure;
[A] and [B] is the concentration of the acid, base form, respectively, of the dye in the sample; and
γA and γB is the activity coefficient of the acid, base form of the dye, respectively, and a function of temperature, pressure, and ionic strength of solution. Equation 2 is more commonly written as:
                              pH          =                                    pK              a              ′                        +                          log              ⁢                                                          ⁢                                                [                  B                  ]                                                  [                  A                  ]                                                                    ,                                  ⁢                  where          ⁢                      :                                              Eq        .                                  ⁢        3                                          pK          a          ′                =                  -                                                    log                10                            ⁡                              (                                                      K                    a                                    ⁢                                                            γ                      A                                                              γ                      B                                                                      )                                      .                                              Eq        .                                  ⁢        4            
Because pKa′ includes the activity coefficients, it is no longer only a function of pressure and temperature, but also a function of ionic strength. Calibration at ambient conditions using standard buffers of known pH is well established. A two-wavelength measurement allows calculation of [B]/[A] and hence the determination of the pH of unknown solutions using Equation 3.
The challenge is in extending this technique to higher temperatures and pressures where there are no standard calibrating buffers. The International Union of Pure and Applied Chemistry (IUPAC) 2002 guidelines for pH measurement using standard electrodes are valid only to 50° C., 1 bar, and ionic strengths below 0.1 gmol/kg (see Buck et al., Measurement of pH Definition, Standards and Procedures—IUPAC Recommendations 2002, Pure and Applied Chemistry, 2002, Vol. 74, Issue 11. At higher temperatures, pressures, and ionic strengths, there are inherent uncertainties associated with liquid junction potentials; because of this uncertainty, currently there are no guidelines for making pH measurements under these conditions. Standard buffer solutions are typically certified at room temperatures. When buffer solutions are heated or their salinity (ionic strength) is changed, their pH values change and as a result they are no longer the original pH certified standards.
Raghuraman et al. in Real-Time Downhole pH Measurement Using Optical Spectroscopy, SPE International Symposium on Oilfield Chemistry, Feb. 2-4, 2005, Houston, Tex., USA, SPE 93057 describe a methodology for calibration and extension of the spectroscopic measurement to higher temperatures and pressures. Standard buffer solutions are simple salts whose chemical equilibria have been reported over a range of temperature, pressure, and salinity conditions. As a first step to calibrating dyes for pH measurements at high-temperature and high-pressure conditions, one could use models of standard salt buffer equilibria to calculate pH at these conditions. Using these pH values for calibration, one can determine pKa′ of various pH-sensitive dyes as a function of temperature (to 150° C.), pressure (to 680 bar) and ionic strength (to 3 mol/kg). The dyes chosen should be ones that can survive and have pH sensitivity under these conditions. Once pKa′ is known, Eq. 3 can be used to calculate pH at any temperature, pressure, and ionic strength by measuring the dye-sample spectra.
Commonly owned Great Britain Patent No. 2,395,555, entitled “Apparatus and Method for Analyzing Downhole Water Chemistry,” incorporated by reference herein in its entirety, teaches a method of using dyes to measure pH in high temperature pressure oil wells at downhole conditions.
Commonly owned United States Patent Application Publication Number 20040128974, filed Sep. 22, 2003, entitled “Determining fluid chemistry of formation fluid by downhole reagent injection spectral analysis,” incorporated herein by reference in its entirety, teaches a method for analyzing formation fluid in earth formation surrounding a borehole that includes storing analytical reagent in a container in a fluids analyzer in a formation tester and moving the formation tester, including the reagent, downhole. Reagent from the reagent container is injected into formation fluid in the flow-line to make a mixture of formation fluid and reagent. The mixture is moved through a spectral analyzer cell in the fluids analyzer to produce a time-series of optical density measurements at a plurality of wavelengths. A characteristic of formation fluid is determined by spectral analysis of the time-series of optical density measurements.
Single dyes are typically sensitive to only about 1.5 units on either side of their pKa. To make measurements over a wide range of pH, one has to either use many single dyes or alternatively use a dye mixture. Commonly owned United States Patent Application Publication Number 20040219064, filed Feb. 19, 2004, entitled “Spectroscopic pH measurement using optimized reagents to extend measurement range,” incorporated herein by reference in its entirety, teaches an indicator mixture that allows pH measurement over a broader range of pH or to a higher accuracy than available using conventional spectroscopic techniques. In particular, the mixture of the present invention is comprised of two or more reagents such that when combined, the reagent mixture is capable of either detecting: (1) a pH range broader or more accurate than the reagent individually, or (2) pH more accurately than the reagents individually. Also disclosed are methods of making and using the mixture. This methodology allows high-accuracy, extended-range pH measurement with a single dye mixture (see FIGS. 1 and 2). Raghuraman et al. in Real-Time Downhole pH Measurement Using Optical Spectroscopy, SPE International Symposium on Oilfield Chemistry, Feb. 2-4, 2005, Houston, Tex., USA, SPE 93057 report pH measurements in high-temperature, pressure oil wells using these techniques and apparatuses.
As mentioned earlier, today there is standard IUPAC recommendation or any spectroscopic method for high-temperature, high-pressure and high-ionic strength measurement of pH in the laboratory. Conventional potentiometric techniques work only to temperatures of 50° C., 1 bar and ionic strength below 0.1 gmol/kg. Thus, there is a need for high-temperature and high-pressure measurements of pH in the laboratory.